The invention relates to a dip-coated ferritic stainless steel sheet usable in the automobile exhaust sector.
Processes are known for improving the wettability of aluminum on metal sheets made up of chromium iron alloys. In European Patent No. 0 467 749, a process is described that eliminates the need to have very pure hydrogen in the aluminum alloying furnace. By preheating the metal sheet at less than 500xc2x0 C. in a non-oxidizing atmosphere containing less than 3% oxygen and heating the metal sheet to a temperature of less than 950xc2x0 C. in a second non-oxidizing atmosphere with a dew-point of less than xe2x88x9240xc2x0 C., preferably less than xe2x88x9250xc2x0 C.; the atmosphere in the annealing furnace and in the hood of the furnace does not have to be formed from pure hydrogen. The metal sheet can be placed in a non-reactive atmosphere, such as a nitrogen or nitrogen and hydrogen atmosphere. The nitrogen atmosphere has less than 20 ppm of oxygen and a dew-point of less than xe2x88x9260xc2x0 C. The metal sheet is cooled off to roughly the temperature of the bath, and it is placed in the molten aluminum bath. An aluminum bath containing silicon reduces to a minimum the alloying layer of the interface and reduces the fragility of the coated metal sheet.
The goal of the present invention is to propose a stainless steel sheet comprising an aluminum coating, usable in the automobile exhaust sector and having improved oxidation resistance characteristics; with the metal sheet coated in this way is also able to be used to produce a catalyzer support strip.
The object of the invention is a dip-coated ferritic stainless steel sheet usable in the automobile exhaust sector, characterized in that it comprises a steel core with the following composition by weight:
10.5% less than chromiumxe2x89xa620%
0%xe2x89xa6aluminumxe2x89xa60.6%
0.003%xe2x89xa6carbonxe2x89xa60.06%
0.003%xe2x89xa6nitrogenxe2x89xa60.03%
0%xe2x89xa6siliconxe2x89xa60.6%
0%xe2x89xa6manganesexe2x89xa60.6%
0%xe2x89xa6sulfurxe2x89xa60.002% iron and impurities inherent in processing, and a metal coating deposited by dipping the strip in a molten metal bath containing in particular aluminum and at least one rare earth element: cerium, lanthanum, praseodymium, neodymium, mixed metal and/or yttrium.
The other characteristics of the invention are:
the steel has less than 18.5% chromium in its composition,
the molten metal bath has a rare earth content, Ce, La, Pr, Nd, mixed metal and/or yttrium, taken alone or in a combination lower than 3.0%.
the coating optionally comprises 6% to 13% silicon, the silicon being introduced into the molten metal bath of aluminum.
the steel comprises less than 0.6% aluminum and less than 0.03% nitrogen, the carbon and nitrogen levels preferably conforming to the relationship: (C+N) less than 0.04%.
the steel additionally comprises stabilizing elements: titanium, zirconium, niobium taken alone or in combination, at a level less than or equal to 0.6%.
the steel additionally has in its composition less than 1.5% copper and/or less than 1% nickel and/or less than 5% molybdenum.
The coated ferritic stainless steel sheet of the invention is used in the automobile exhaust sector to manufacture components of said vehicle""s exhaust pipe: tubes, mufflers sheaths and catalyzer cones.
The invention also relates to the use of coated ferrite stainless steel sheets in the automobile exhaust sector to manufacture a catalyzer support strip.
The purpose of the invention is also a process to produce a metal supporting strip for catalyzers from the coated metal sheet, a process in which:
the coated metal sheet is cold-rolled in at least one rolling step to obtain a steel strip,
the steel strip is annealed in a non-oxidizing atmosphere at a temperature of 600xc2x0 C. to 1,200xc2x0 C., preferably 800xc2x0 C. to 1,000xc2x0 C. so as to cause the metal of the coating to diffuse into the steel to form the strip.
The other characteristics of the invention are:
the coated steel sheet is cold-rolled in two steps which may comprise an intermediate annealing, the second step being carried out with a rolling rate of 20% or more, ensuring final roughness Ra of less than 0.25 xcexcm.
the coated steel sheet is cold-rolled until reaching a final thickness of 30 to 50 xcexcm.
According to another form of the process for producing a metal supporting strip for catalyzers from the coated metal sheet:
the coated sheet is annealed in a non-oxidizing atmosphere at a temperature of 600xc2x0 C. to 1,200xc2x0 C., preferably 800xc2x0 C. to 1,000xc2x0 C., so as to cause the metal of the coating to diffuse into the steel.
the metal sheet subjected to diffusion is rolled in at least one rolling step to obtain a strip.
The characteristics according to the forms of the invention are:
the annealings are carried out in a hydrogen atmosphere with a dew point below xe2x88x9230xc2x0 C., either statically or continuously.
the final, softening annealing is carried out continuously at a temperature of 600xc2x0 C. to 1,200xc2x0 C., preferably 800xc2x0 C. to 1,000xc2x0 C.
The invention also relates to a strip usable in particular as an auto exhaust catalyzer support, characterized in that it has the following composition by weight:
10.5%xe2x89xa6chromiumxe2x89xa620%
0%xe2x89xa6aluminumxe2x89xa68.0%
0.003%xe2x89xa6carbonxe2x89xa60.06%
0.003%xe2x89xa6nitrogenxe2x89xa60.03%
0%xe2x89xa6siliconxe2x89xa60.6%
0%xe2x89xa6manganesexe2x89xa60.6%
0%xe2x89xa6sulfurxe2x89xa60.002%
0.001%xe2x89xa6rare earth, Ce, La, Pr, Nd, mixed metal and/or yttrium, taken alone or in combination xe2x89xa61.0%, iron and impurities inherent in processing and in which the chromium and aluminum levels conform to the relationship %Cr/%Al less than 5.